Precise frequency control and high spectral purity are important for many laser applications including high-resolution spectroscopy, precision interferometric measurement, frequency-selective optical data storage, and optical communications. Various methods and apparatus have been employed in attempts to achieve such frequency control.
In one approach, feedback is used to minimize the frequency offset from an absolute frequency reference, such as an atomic or molecular absorption line. See, e.g., M. W. Hamilton, Contemporary Phys. 30, 21, p. 25 (1989). But such methods depend upon the availability of a reference substance having an absorption line at the desired frequency, and thus cannot stabilize a laser at an arbitrary frequency. Further, the range and speed of tuning are limited because of the large magnetic fields required to frequency-shift the absorption resonances of the atomic or molecular systems of the reference substance. Moreover, a small frequency modulation of the laser is typically required in order to generate the feedback signal, placing an upper limit on the spectral purity attainable.
In another approach, a relatively stable "master" laser is used to control the frequency of an unstable "slave" laser. Such methods require the added complexity of a second laser. More significantly, the master laser must have an appropriate frequency relative to the frequency at which the slave laser is to be stabilized, and relatively stable lasers having such an appropriate frequency may not be readily available. Furthermore, the resulting stability is only as good as that of the master laser.
In yet another third approach, a reference cavity may be used to stabilize a laser. See. e.g., M. Ohtsu, K. Nakagawa, M. Kuorogi, W. Wang, J. Appl. Phys. 73, 1 (1993) and R. W. P. Drever, J. L. Hall, F. V. Kowalski, J. Hough, G. M. Ford, A. J. Munley, H. Ward, AppI. Phys. B 31, 97 (1983). These methods allow tuning at arbitrary frequencies, but the tuning speed is slow because the cavity length must be varied, typically by mechanical means. The stability achieved is limited to the stability of the reference cavity.
In still another approach, two self-referenced interferometric schemes have been proposed to control the frequency of a laser (see J. Hall, U.S. Pat. No. 4,856,009 and Y. T. Chen, Appl. Opt. 28, 2017 (1989)). While these methods allow the initial stabilization of a laser at an arbitrary frequency, subsequent stabilizations can occur only at a set of discrete frequencies relative to the initial frequency. More significantly, the feedback electronics must be actively adjusted in order to stabilize the laser at frequencies other than the initial frequency.